The present disclosure generally relates to apparatuses and methods for measuring and controlling the viscosities of liquids.
Viscosity measurement of liquids is a procedure used in the characterization of many liquid products including polymers and polymer melts. The testing is typically performed off-line on a small scale in a laboratory setting. Samples of the polymer melt are periodically withdrawn from the main process stream during the manufacturing process and carried to the laboratory to be tested and characterized. The test results are then used to tweak and adjust the manufacturing process to obtain a targeted polymer melt quality. Data obtained in this manner is not representative of the ongoing manufacturing process since a significant time delay in obtaining the data exists.
More recently, efforts have been directed to reducing the response time by measuring polymer melt properties during the manufacturing process itself in order to provide better control over the quality of the polymer melt. Measurement during the manufacturing process, however, requires equipment that is relatively easy to use, maintain, and rugged enough to withstand the operating conditions to which the equipment will be exposed. In order to be effective, the equipment must be responsive, and must avoid disturbing the manufacturing process being monitored.
Among the more successful rheometers employed during the manufacturing process are capillary rheometers that divert a portion of the polymer melt from the main stream of molten plastic, conduct measurements on the diverted polymer melt, and then simply purge the polymer melt out to the atmosphere. These types of rheometers are often referred to as on-line rheometers. Other types of capillary rheometers have been developed in which the diverted melt is returned to the main stream, thereby eliminating waste and the additional steps associated with the purge stream. These types of capillary rheometers are often referred to as in-line rheometers.
Capillary rheometers usually employ a first metering pump, such as a gear pump, to feed a capillary passage with a controlled flow of the diverted polymer melt. In the case of in-line rheometers, a second metering pump is employed to return the diverted melt to the main stream. In capillary viscosity measurement, a pressure drop of a liquid flowing through the capillary is used to measure its viscosity. Typically, the capillary has a very small internal diameter, such as 3 mm or less. When a liquid exhibits a high resistance to flow through the capillary, its viscosity is high, and vice versa.
In using the capillary measurement technique, the equation for determining absolute viscosity at a given temperature, which is known as the Poiseuille equation, is shown in equation (I):                     η        =                              π            ⁢                          xe2x80x83                        ⁢                          (                              Δ                ⁢                                  xe2x80x83                                ⁢                P                            )                        ⁢                          xe2x80x83                        ⁢                          r              4                                            8            ⁢            LF                                              (        I        )            
where xcex7 is the viscosity of the liquid in poise or grams per second-centimeter, and (xcex94P) is the pressure drop across the capillary in dynes-second per square centimeter; F is the flow rate through the capillary in cubic centimeters per second; L is the length of the capillary in centimeters, and r is the internal radius of the capillary in centimeters.
The actual process of measuring viscosity offline with capillary rheometers is time consuming. Each measurement of the drop time through the capillary requires from three to fifteen minutes, depending on the viscosity of the polymer melt, and must be duplicated in order to obtain a value that can be relied upon. If the second drop time is not close enough to the first, a third, or even fourth drop time must be obtained. The complete viscosity determination, from sample equilibration to measurement, and then through to viscometer cleaning, typically takes from twenty to forty minutes. As a result, the data obtained from capillary rheometers is not representative of the actual ongoing manufacturing conditions.
Non-capillary viscometers have also been developed to measure the viscosity of multiple liquid samples as well as melt flows. Such viscometers include rotation viscometers. A typical rotation viscometer comprises two concentric cylinders, the inner or outer cylinder being rotated in or rotated around the fixed outer or inner cylinder. With such a viscometer, the test liquid is placed between two cylinders and either of the cylinders is rotated around its axis. However, rotation viscometers have several disadvantages. Several measurements must be made on the same liquid sample at different shear stresses. Calculations of viscosity from these devices are troublesome and lead to noticeable errors as they require graphical differentiation of logarithmic values. Special and unstable flows take place at high rotating rates. Furthermore, the devices, which are difficult to clean, must be cleaned after each measurement. Finally, rotational viscometers do not easily render themselves to automation.
Still other non-capillary viscometers have been developed that measure viscosity using different scientific principles. For instance, differential viscometers, such as those made by the Viscotek Company, are based on a fluid analog of the wheatstone bridge, and allow solvents to flow continuously through a bridge network. The differential pressure across the bridge is zero until the sample solution in a reservoir is injected into one of the capillaries. The differential pressure begins to rise until it reaches a steady state value proportional to the specific viscosity of the solution. The viscosity is then calculated from an extensive series of equations. Such a device is limited in its viscosity range and measures viscosity by comparison.
Finally, other non-capillary type viscometers have been developed that calculate viscosity by first determining the flow rate of a liquid passing from a vessel at a given pressure to a vessel at a lower pressure and by the change rate of the internal vessel pressure. Such devices calculate the viscosity of the liquid from shear stress and shear rate by using a lengthy array of calculations. While each of these viscometers has proven superior to the bulb-type capillary viscometer, they still require extensive time to operate and are often not fully automated. Many of these types of devices are limited in their viscosity measurement range.
Accordingly, there remains a need for a method of determining the real-time viscosity and rheology of a polymer melt.
Disclosed herein is a method for measuring, in real time, rheological properties of a polymer melt. The method comprises flowing a polymer melt from a first reactor to a device through a main conduit at a predetermined flow rate, wherein substantially all of the polymer melt flowing from the reactor to the device flows through the main conduit; measuring a first pressure of the polymer melt at a first location in the main conduit; measuring a second pressure of the polymer melt at a second location in the main conduit downstream from the first location; measuring a temperature of the polymer melt at a third location intermediate to the first location and the second location; and calculating a rheological property of the polymer melt as it flows between the first and second locations of the main conduit.
An apparatus for measuring a property of a polymer melt comprises means for containing a polymer melt under pressure; means for flowing the polymer melt at a defined flow rate through a conduit from the containing means to a device, wherein substantially all of the polymer melt flowing into the device flows through the conduit; pressure sensing means for measuring a first pressure of the polymer melt at a first location in the conduit and producing a first pressure signal, and for measuring a second pressure at a second location in the conduit and producing a second pressure signal, wherein the second location is downstream from the first location; temperature sensing means disposed between the first and second locations for measuring a temperature of the polymer melt and producing a temperature signal; and processor means responsive to the temperature and pressure signals for determining a rheology property of the polymer melt, wherein the processor means is in operable communication with the pressure sensing means, the temperature sensing means and the means for flowing the polymer melt through the conduit.
The rheological property is selected from the group consisting of mass flow rate (xe2x80x9cMFRxe2x80x9d) and mass volume flow rate (xe2x80x9cMVRxe2x80x9d). MFR is calculated from a formula comprising:   MFR  =            ρ      ⁡              [                              Qc            ⁢                          xe2x80x83                        ⁢                          exp                              (                                                      -                    D                                    /                  T                                )                                                                        (                                                [                                      0.5                    ⁢                                          xe2x80x83                                        ⁢                                          (                                              1                        -                        a                                            )                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                              P1                        +                                                  (                                                      P2                            -                            b                                                    )                                                                    )                                                        ]                                a                            )                                      (                              1                +                n                            )                                      ]              +    d  
P1 is a measurement of the pressure at the first conduit location, P2 is a measurement of the pressure at the second location, T is a measurement of the temperature at the third location, a, b, c, d, D, n, xcfx81, are constants, and Q is the defined flow rate of the polymer melt.
MVR is calculated from a formula comprising:   MVR  =                    Qc        ⁢                  xe2x80x83                ⁢                  exp                      (                                          -                D                            /              T                        )                                                (                                    [                              0.5                ⁢                                  xe2x80x83                                ⁢                                  (                                      1                    -                    a                                    )                                ⁢                                  xe2x80x83                                ⁢                                  (                                      P1                    +                                          (                                              P2                        -                        b                                            )                                                        )                                            ]                        a                    )                          (                      1            +            n                    )                      +    d  
P1 is a measurement of the pressure at the first conduit location, P2 is a measurement of the pressure at the second location, T is a measurement of the temperature at the third location, a, b, c, d, D, n, are constants, and Q is the defined flow rate of the polymer melt.